A honeybee colony will issue what is called a 'swarm'. The swarm contains the original colony's queen along with about half of the worker population, including most of the field bees.

This cluster is in the early stage of forming on a low hanging branch. Soon, all the members of this swarm will alight.

The bees have gathered in their temporary stop over. While here, the colony sends out about a dozen or more scouts to search for an appropriate, sheltered, permanent location to build a new home. The scouts will come back and communicate their chosen location, as well as their enthusiam for that location. Typically, within 24 hours the swarm chooses one of the advertised locations and moves on.

Swarming is the method honeybees naturally propogate themselves, at the colony level.

This swarm gathered in a pine tree about 12 feet up, around noon. Honeybee colonies typically issue a swarm during favorable weather, between late morning and mid afternoon. This medium super (bee hive box) has a solid plywood bottom, which is kept handy just for these opportunities.
Swarms are not threatening.

The bees have gorged themselves on honey for the trip, they are homeless, have no honey or brood to defend and are focused on their mission to find a new permanent location.

Check out the video clips of capturing and hive this swarm here.

This looked like a good cover for a swarm box, but didn't work. In order to leave the colony, the swarming queen stops egg production in order to reduce her body weight for flight. She is then small enough to fit through this bars in a queen excluder. The swarm left this box and started their journey over.
Good bye bees.
There's always another swarm
The season, weather, and swarm location made this swarm relatively easy to collect and hive.